The concept of nanofluids, i.e., nanoparticle-fluid dispersions, was introduced in the mid 1950's at the Argonne National Laboratory. Compared with millimeter- or micrometer-sized particle suspensions, nanofluids generally possess improved long term stability, much higher surface area, as well as improved mechanical, thermal and rheological properties. However, recent research efforts on nanofluids have mainly been focused on the preparation and evaluation of water or ethylene glycol (EG)-based nanofluids while reports of the synthesis of oil-based nanofluids are relatively uncommon.
Many nanofluids offer the potential for significant improvements in two distinct properties of interest to this proposal, namely tribological (friction and wear) and thermal properties. For example, regarding tribological properties, in the late 1960's, mineral oils with dispersed molybdenum disulfide (MoS2) particles with an average size of 0.4 μm were tested and improved load carrying capacity and lower wear rate were reported. The improved tribological properties were attributed to strongly adhering and reformable deposits of flakes of the solid lubricant MoS2 which provided a low shearing localized film between rubbing surfaces.
In recent studies, marked improvements in tribological properties of oils with dispersed inorganic fullerene-like (IF) 150-260 nm molybdenum disulfide (MoS2) and 100-120 nm tungsten disulfide (WS2) nanoparticles have been reported. The improvement was attributed to chemical stability of IF nanoparticles that resulted in reduced oxidation. The antiwear and low friction behavior of a variety of nanoparticle dispersions including metallic oxides such as copper, zinc, zirconium, and titanium oxides and borates including titanium, lanthanum, zinc and ferrous borates and sulfides such as molybdenum and tungsten sulfides when dispersed in lubricants have also been reported. Nanoparticle characteristics such as size, shape, and concentration are shown to influence the tribological properties.
There have been several mechanisms contemplated in the literature by which dispersed nanoparticles in lubricants result in lower friction and wear. These mechanisms include: formation of a transferred solid lubricant film from nanoparticles under the contact pressure, rolling of spherical nanoparticles in the contact zone, reducing asperity contact by filling the valleys of contacting surfaces, and shearing of nanoparticles at the interface without the formation of an adhered film.
A new mechanism for the role of solid lubricant nanoparticles was recently proposed. According to the proposed mechanism, one role of solid lubricant nanoparticles in oils and greases is to break apart the wear agglomerate that is commonly formed at the sliding interface. The wear agglomerate, sometimes referred to as the transferred film, is normally adhered to the harder surface. The entrapment of the wear agglomerate reduces the contact area which in turn causes the normal contact pressure to be increased. Therefore, the plowing of the mating surface by the wear agglomerate is enhanced. The enhanced plowing increases friction and wear. The wear debris agglomeration process and some factors that affect it are discussed in the literature.
One of the reasons for the significant attention to nanofluids has been due to their enhanced thermal characteristics. From heat transfer theories, for a constant Nusselt number, the convective heat transfer coefficient is directly proportional to the thermal conductivity. With this observation, many researchers have focused on the thermal conductivity of nanofluids.
Some nanoparticles are known to be very thermal conductive. It has been shown that the dispersion of nanoparticles in fluids can improve the suspension's effective thermal conductivity. For instance, some experimental studies revealed that even for a very small percentage of 0.1-0.5 of metallic or oxide nanoparticles, the effective thermal conductivity of the dispersion can be increased by as much as 5-60%. Also, the effective thermal conductivity of ethylene glycol (EG) is increased by up to 40% when a 0.3 volumetric percent of copper nanoparticles of mean diameter less than 10 nm are dispersed in it. Use of dispersions of thermally conductive nanoparticle in combination with lubricating nanoparticles has heretofore been unknown to provide superior lubrication with superior heat dissipation to provide lubricants which not only have superior lubrication properties, but also mitigate or eliminate hot sports which can cause excessive part wear.
U.S. Publication No. 2011/0003721 to Hong et al. describes a nanofluid which comprises a thermal transfer fluid and carbon nanoparticle tubes as a part of lubricating compositions. Hong et al. describe a nanogrease using carbon nanotubes as a solid heat transfer medium to enhance thermal conductivity and high temperature resistance. However, Hong et al. fail to address hybrid integrated nanoparticles, such as formed from multiple nanoparticle components, which effect multiple functionalities of lubrication and heat dissipation.